Dynamical Response of Nonlinear Optical Anisotropy in a Tin Sulfide Crystal under Ultrafast Photoexcitation.

Analogous to black phosphorus, SnS processes folded structure that shows a strongly anisotropic optical absorption. Herein, by using ultrafast two-color pump and probe spectroscopy, the azimuthal angle dependence of nonlinear optical anisotropy in SnS is investigated. After 390 nm photoexcitation, the reflectivity of the 780 nm probe beam is first reduced significantly, followed by a complex alternation with the rotation of the sample along the c-axis. The relaxation of reflectivity consisted of two components: a 1-3 ps fast process that shows azimuthal angle and pump fluence dependence, which arises from electron-phonon coupling. The slow process shows strongly azimuthal angle dependence, which arises from the recovery of a photoinduced structural change, i.e., from the photoinduced metastable state with Cmcm-like symmetry to the initial state with Pnma symmetry. In addition, a coherent acoustic phonon with a frequency of 40 GHz is also identified, which originates from the temperature gradient-induced strain wave in the SnS crystal.

Ultrafast Dynamics of Defect-Assisted Auger Process in PdSe2 Films: Synergistic Interaction between Defect Trapping and Auger Effect.

By using optical pump and terahertz probe spectroscopy, we have investigated the photocarrier dynamics in PdSe2 films with different thicknesses. The experimental results reveal that the photocarrier relaxation consists of two components: a fast component of 2.5 ps that shows the layer-thickness independence and a slow component that has typical lifetime of 7.3 ps decreasing with the layer thickness. Interestingly, the relaxation times for both fast and slow components exhibited both pump fluence and temperature independence, which suggests that synergistic interactions between defect trapping and Auger effect dominate the photocarrier dynamics in PdSe2 films. A model involving a defect-assisted Auger process is proposed, which can reproduce the experimental results well. The fitting results reveal that the layer-dependent lifetime is determined by the defect density rather than carrier occupancy rate after photoexcitation. Our results underscore the interplay between the Auger process and defects in two-dimensional semiconductors.

Optically controlled ultrafast terahertz switching in wafer scale PtSe2 thin films.

In this study, we have reported a newly ultrafast optically modulated terahertz (THz) switch based on the transition metal dichalcogenide (TMD) material platinum diselenide (${{\rm PtSe}_2}$) with different thicknesses. The high-quality ${{\rm PtSe}_2}$ thin films with centimeter scale are fabricated on sapphire substrate by the chemical vapor deposition method. The optical pump and THz probe (OPTP) spectroscopy reveals that the THz response of the thin films is as fast as ${\sim} 2.0 \; {\rm ps}$ after photoexcitation of a 780 nm pulse. Interestingly, we found that the THz response time of the ${{\rm PtSe}_2}$ semimetal phase is faster than that of the semiconducting phase. In addition, the THz response time becomes faster when increasing the film thickness for the semimetal phase ${{\rm PtSe}_2}$, while for the semiconducting phase, the response time becomes slower with film thickness. Moreover, degenerate optical pump and optical probe spectroscopy (OPOP) demonstrated that the ultrafast photoinduced negative absorption (photoinduced bleaching) occurs after photoexcitation of 780 nm, and the subsequent recovery consists of two relaxation processes: the fast component with more than 85% of weight has a lifetime of ${\sim}{1.5}\;{\rm ps}$ for semiconducting-phase films and less than 1 ps for the semimetal phase, similar to the response time obtained from OPTP measurement. The slow component with less than 15% of weight has a lifetime of a few hundred picoseconds. The subpicosecond response time observed in both OPTP and OPOP is ascribed to the carrier trapping by defect states, and the slow relaxation process appearing in OPOP arises from the defect state relevant relaxation that is insensitive to the THz photoconductivity due to the frozen carrier mobility in defect states. Our experimental results demonstrate a new application of TMD materials such as ${{\rm PtSe}_2}$ in THz technology, for instance, the design and fabrication of THz modulators and THz switches.

Surface-State Assisted Carrier Recombination and Optical Nonlinearities in Bulk to 2D Nonlayered PtS.

Cooperite, or platinum sulfide (PtS), is a rare mineral that generally exists as microscale, irregularly shaped crystallites. The presence of impurities, in both naturally occurring and synthesized samples, has hindered the study of its optical properties in the past. In this work, we prepare large-scale, uniform PtS films in bulk to two-dimensional form through the thermally assisted conversion method. An abnormal trend is observed in linear spectral studies whereby the optical bandgap narrows as the film thickness decreases. A model based on the continuous distribution of carriers in real space, which can be regarded as a quantum well normal to the plane, is used to describe the thickness-dependent carrier recombination phenomenon. In the nonlinear optical measurements, PtS exhibits ultrafast saturable absorption and self-defocusing properties in the visible region, which are dominated by the resonant electronic nonlinearities.

Photonic-crystal-based broadband graphene saturable absorber.

The enhanced saturable absorption (SA) of a one-dimensional (1D) photonic crystal (PC) made from polymers and graphene composites by spin coating is observed. It shows obvious bandgaps at two wavelengths in transmittance. Femtosecond Z-scan measurement at 515 nm and 1030 nm reveals a distinct enhancement in the effective nonlinear absorption coefficient ßeff for graphene nanoflakes embedded in the PC, when compared with the bulk graphene-polymer composite. The effect is studied in a wide range of laser intensities. Graphene inclusion into a 1D PC remarkably decreases the SA threshold and saturation intensity, providing a desired solution for an advanced all-optical laser mode-locking device.

Bacterially synthesized tellurium nanostructures for broadband ultrafast nonlinear optical applications.

Elementary tellurium is currently of great interest as an element with potential promise in nano-technology applications because of the recent discovery regarding its three two-dimensional phases and the existence of Weyl nodes around its Femi level. Here, we report on the unique nano-photonic properties of elemental tellurium particles [Te(0)], as harvest from a culture of a tellurium-oxyanion respiring bacteria. The bacterially-formed nano-crystals prove effective in the photonic applications tested compared to the chemically-formed nano-materials, suggesting a unique and environmentally friendly route of synthesis. Nonlinear optical measurements of this material reveal the strong saturable absorption and nonlinear optical extinctions induced by Mie scattering over broad temporal and wavelength ranges. In both cases, Te-nanoparticles exhibit superior optical nonlinearity compared to graphene. We demonstrate that biological tellurium can be used for a variety of photonic applications which include their proof-of-concept for employment as ultrafast mode-lockers and all-optical switches.

Enhanced two-photon absorption and two-photon luminescence in monolayer MoS2 and WS2 by defect repairing.

In this work, we investigated the nonlinear optical properties of monolayer MoS2 and WS2 modulated by defect engineering via chemical treatment. The results demonstrate that the two-photon luminescence (TPL) and two-photon absorption (TPA) coefficient were remarkably improved after the repair of sulfur vacancies for both monolayer MoS2 and WS2. After the chemical treatment, the nonradiative relaxation path dominant in pristine monolayer MoS2 is significantly alleviated, resulting in enhanced TPL. Our work affords an effective way to tailor the nonlinear absorption, luminescence and relaxation properties of sulfur-based two-dimensional metal dichalcogenides by defect engineering.

Tailoring the nonlinear optical performance of two-dimensional MoS2 nanofilms via defect engineering.

Defect engineering plays a key role in determining the catalytic and optical properties of two-dimensional (2D) materials such as molybdenum disulfide (MoS2) in their practical applications in optical and photonic devices. Here, we report a direct strategy for the fabrication of wafer-scale 2D MoS2 nanofilms with tunable sulfur (S) vacancies and crystallinity by a modified solvothermal method via a polyelectrolyte-assisted annealing process. Our results demonstrate that the S vacancies in MoS2 nanofilms can induce saturable absorption (SA) in MoS2 by introducing new energy bands within the band gap of MoS2, and the crystallinity has a significant effect on the two-photon absorption (TPA) coefficient of MoS2 nanofilms. The SA responses in MoS2 will gradually dominate the nonlinear optical (NLO) behavior of MoS2 with a lower saturable intensity along with increasing the S vacancies. The TPA coefficient of the MoS2 nanofilms with increased crystallinity is improved to (4.3 ± 0.5) × 102 cm GW-1 on increasing the crystallinity of MoS2 films, over four times larger than that of their counterpart with relatively low crystallinity. Additionally, the damage threshold of MoS2 nanofilms after polyelectrolyte-assisted annealing treatment is greatly improved to ∼74.1 GW cm-2 compared to ∼32.6 GW cm-2 of their counterpart with few S vacancies and relatively low crystallinity, due to the increased crystallinity and partial oxidation of MoS2. This work sheds light on how the defects tailor the nonlinear optical properties of 2D MoS2 nanofilms and affords an effective strategy for defect engineering via a polyelectrolyte-assisted annealing process, which can be applied to other 2D transition metal dichalcogenides.

Intensity-dependent nonlinear refraction of antimonene dispersions in the visible and near-infrared region.

Antimonene is a stable 2D allotrope of antimony that is predicted to have a direct bandgap, high third-order optical nonlinear susceptibility, and high electron mobility. These properties give it huge potential applications in photonics and optoelectronics. However, the nonlinear refractive response of antimonene dispersions, which is the key to nonlinear refraction-based devices, has not been fully investigated. In this work, we investigated the optical nonlinearities of the antimonene dispersions by spatial self-phase modulation (SSPM) at 405, 785, and 1064 nm wavelengths. The SSPM rings were observed at 405, 785, and 1064 nm, implying the broadband nonlinear optical response of antimonene dispersions from visible to near-infrared. The effective nonlinear refractive index, n2, and the third-order susceptibility, χ(3), of the antimonene dispersion were measured to be ∼10-5 cm2 W-1 and ∼10-8 esu, respectively. Furthermore, the nonlinearity of antimonene was demonstrated to be tuneable by the laser intensities. The relative change of the nonlinear refractive index, Δn2e/n2e, was observed to range from 14% to 63% for different intensities. Our results will be helpful for the photonic applications of antimonene in a broadband wavelength range, such as optical modulators and switchers.

MoS2 /Carbon Nanotube Core-Shell Nanocomposites for Enhanced Nonlinear Optical Performance.

Nanocomposites of layered MoS2 and multi-walled carbon nanotubes (CNTs) with core-shell structure were prepared by a simple solvothermal method. The formation of MoS2 nanosheets on the surface of coaxial CNTs has been confirmed by scanning electron microscopy, transmission electron microscopy, absorption spectrum, Raman spectroscopy, and X-ray photoelectron spectroscopy. Enhanced third-order nonlinear optical performances were observed for both femtosecond and nanosecond laser pulses over a broad wavelength range from the visible to the near infrared, compared to those of MoS2 and CNTs alone. The enhancement can be ascribed to the strong coupling effect and the photoinduced charge transfer between MoS2 and CNTs. This work affords an efficient way to fabricate novel CNTs based nanocomposites for enhanced nonlinear light-matter interaction. The versatile nonlinear properties imply a huge potential of the nanocomposites in the development of nanophotonic devices, such as mode-lockers, optical limiters, or optical switches.

Slow and fast absorption saturation of black phosphorus: experiment and modelling.

According to the excited-state decay time, a slow saturable absorber model was employed to obtain the ground-state absorption and excited-state absorption cross sections σgs and σes of black phosphorus (BP) nanosheets, which are (1.25 ± 0.07) × 10-16 cm2 and (2.97 ± 0.21) × 10-17 cm2 at 515 nm, and (5.95 ± 0.17) × 10-18 cm2 and (5.19 ± 0.71) × 10-19 cm2 at 1030 nm, respectively. In comparison, the σgs and σes of MoS2 and graphene were also obtained with the same model. The ratio σes/σgs of BP is the smallest (only 0.24 ± 0.03 at 515 nm and 0.09 ± 0.01 at 1030 nm) among the three two-dimensional layered materials both at 515 nm and 1030 nm. Relatively large ground-state transition probability and weak loss of excited-state absorption result in good saturable absorption performance. In comparison with MoS2 and graphene, the much lower saturation intensity of BP can be well explained from this perspective.

Colloidal quantum-dot-based silica gel glass: two-photon absorption, emission, and quenching mechanism.

Two-photon (TP) three-dimensional solid matrices have potential applications in high density optical data reading and storage, infrared-pumped visible displays, lasers, etc. Such technologies will benefit greatly from the advantageous properties of TP materials including tunable emission wavelength, photostability, and simple chemical processing. Here, this ideal TP solid is made possible by using a facile sol-gel process to engineer colloid quantum dots into silica gel glass. Characterization using an open-aperture Z-scan technique shows that the solid matrices exhibited significant TP optical properties with a TP absorption coefficient of (9.41 ± 0.39) × 10(-2) cm GW(-1) and a third-order nonlinear figure of merit of (7.30 ± 0.30) × 10(-14) esu cm. In addition, the dependence of the TP properties on high-temperature thermal treatment is studied in detail to obtain a clear insight for practical applications. The results illustrate that the sample can maintain stable TP performance below the synthesis temperature of the CdTe/CdS colloidal quantum dots. Furthermore, the mechanisms for thermal quenching of photoluminescence under different temperature regimes are clarified as a function of the composition.

Dispersion of nonlinear refractive index in layered WS2 and WSe2 semiconductor films induced by two-photon absorption.

Both the nonlinear absorption and nonlinear refraction properties of WS2 and WSe2 semiconductor films have been characterized by using Z-scan technique with femtosecond pulses at the wavelength of 1040 nm. It is found that these films have two-photon absorption response with the nonlinear absorption coefficient of ∼103 cm GW-1, and a dispersion of nonlinear refractive index in the WS2 films that translated from positive in the monolayer to negative in bulk materials.

Direct synthesis of large-scale hierarchical MoS2 films nanostructured with orthogonally oriented vertically and horizontally aligned layers.

Hierarchical MoS2 thin films nanostructured with orthogonally oriented vertically and horizontally aligned layers were designed and excellent passive Q-switching behavior in a fiber laser was demonstrated. A special solvothermal system containing a small amount of water was applied to synthesize such hierarchical MoS2 nanofilms, in which the reaction rate is carefully controlled by the diffusion rate of the sulfur precursor. Wafer-scale MoS2 thin films with hierarchical structures are formed on various substrates. Moreover, the hierarchical MoS2 thin films consisting of both vertical and horizontal layers can be tuned to possess only horizontally aligned layers by controlling the solvothermal time. To show the potential application proof-of-concept, the nonlinear optical performance of the hierarchical MoS2 was investigated. Superior passive Q-switching behavior in a fiber laser with a minimum pulse width of 2.2 µs was observed.

Optical identification of layered MoS2 via the characteristic matrix method.

It is demonstrated that the characteristic matrix method is effective and reliable for the optical identification of two-dimensional layered nanomaterials on different substrates. By using this method, the authors calculate the reflectivity and optical contrast of layered MoS2 crystallites prepared on quartz by chemical vapor deposition. It is found that the measured pixel intensity of an MoS2 optical image under continuous spectrum light is proportional to the calculated reflectivity, and that the theoretical optical contrast agrees well with the experimental results. This work provides a new way for the calculation of the optical contrast of the 2D nanomaterials and layered heterostructures on various substrates.

Correction: Direct synthesis of large-scale hierarchical MoS2 films nanostructured with orthogonally oriented vertically and horizontally aligned layers.

Correction for 'Direct synthesis of large-scale hierarchical MoS2 films nanostructured with orthogonally oriented vertically and horizontally aligned layers' by Xiaoyan Zhang et al., Nanoscale, 2016, DOI: 10.1039/c5nr05938k.

Liquid exfoliation of solvent-stabilized few-layer black phosphorus for applications beyond electronics.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement.

Optical Limiting and Theoretical Modelling of Layered Transition Metal Dichalcogenide Nanosheets.

Nonlinear optical property of transition metal dichalcogenide (TMDC) nanosheet dispersions, including MoS2, MoSe2, WS2, and WSe2, was performed by using Z-scan technique with ns pulsed laser at 1064 nm and 532 nm. The results demonstrate that the TMDC dispersions exhibit significant optical limiting response at 1064 nm due to nonlinear scattering, in contrast to the combined effect of both saturable absorption and nonlinear scattering at 532 nm. Selenium compounds show better optical limiting performance than that of the sulfides in the near infrared. A liquid dispersion system based theoretical modelling is proposed to estimate the number density of the nanosheet dispersions, the relationship between incident laser fluence and the size of the laser generated micro-bubbles, and hence the Mie scattering-induced broadband optical limiting behavior in the TMDC dispersions.

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films.

The optical nonlinearity of WS2 and MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 films with 1-3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10(4) cm/GW. TPA saturation was observed for the WS2 film with 1-3 layers and for the MoS2 film with 25-27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.

Saturable absorption behavior of free-standing graphene polymer composite films over broad wavelength and time ranges.

A comparative research on saturable absorption (SA) behavior dependence on wavelength and pulse duration was performed for graphene polymer composites. Free-standing graphene-polyvinyl alcohol (PVA) composite films were fabricated by using solution cast method in combination of liquid phase exfoliation. SA responses were observed by using an open-aperture Z-scan technique for 340 fs pulses at 1030 nm and 515 nm from a mode-locked fiber laser, and 6 ns pulses at 1064 nm and 532 nm from a Q-switched Nd:YAG laser. The graphene films possess better SA property, i.e., larger SA coefficient and figure of merit (FOM), and lower saturation intensity I(s), for ns pulses than that for fs pulses at the similar near infrared (NIR) wavelength. For fs pulses, the films show better SA response at 1030 nm than that at 515 nm. By employing slow and fast SA modelling, the excited state and ground state absorption cross sections were estimated to be ~10(-17) cm(2), and the ratio was ~0.6 at NIR for both fs and ns pulses.